CHIPPING, SCREENING AND CLEANING

Debarked logs intended for c refiner pulping need to be chipped inlD pieces of uniform size, and so do sdiII residuals 'K except those produced by a headrig). In the case of chemical wood would otherwise take too long ID impregnated with chemicals and hca digester, and its outside would be (i.e. the lignin-dissolving chemicals started to attack the cellulose and fibers) while its inside was still und was still a chunk of unseparated fibcnt case of refiner mechanical pulping.

The chip size used in practice is of magnitude roughly midway between extremes. The most common range sions for mill-cut chips is: length al between % and 11,4 inches;thickness. inch; and width, variable.This go to have developed for the cooking by acid sulfite liquors, in which penetration in the grain direction is times faster than in the cross-grain . This may be due uncertainty about all the factors include the moisture content and fissures in the chip), in part to the producing chips controlled in diInelDlii. than length, and in part to advantage of having but one size of pulping operations.

Disc Chippers

The means used to subdivide logs must be lpid and cheap - for very large quantities are .nvolved - and yet must produce a relatively .Uniform-sized product with minimum damage and debris (see Process Summary 8). The principle universally used today is to produce each piece with a single blow of a sharp steel knife - i.e. to chip the log. There are two basic types of chipping action which differ in the degree to which they can control chip dimenions.

The first type exerts positive control only ver length. The knife blade, travelling in a :lfection inclined at 30 to 40 from the long zxis, strikes across this axis at a fixed advance 'om the last cut. This fixed advance :dines the length of the chip, i.e. in the grain : erection.Chip thickness, though not directly controlled, can be manipulated to a certain extent. Chip also splits at intervals along planes at right angles to the plane of the blade but perpendicular to its edge. The distance between these splits is the width of the chip. ahe width is normally greater than the thickness, but it is extremely variable.

The commonest equipment using this kind of action is the disc chipper. This consists basically of a rotating disc, 4 to 10 feet in diameter, on which some 4 to 20 knives are mounted radially. Several types of knife arrangement and mounting are used, of which the most common are : Norman design: in which the wear (face) plates of the disc are made so that at any given radius, the slope of the wear plate is a constant from one knife to the next chip slot. Therefore the slope will be larger at a small radius than a large one. The chipper knives are ground so that the clearance

Process Summary 8

Operation ChippingFunctionReduce stems, logs and log segments, wood product residuals

and other pulpable wood waste into pieces of relatively uniform

size called "chips" Equipment optionsDiscV-drum DrumDescription Size

Flat disc with knives mounted radially Disc dia. 3-12 ft; 3-

15 knives; 50-1000 HP drive

Drum" made from two conical segments Dia, 2-10 ft; 6-20 knives; 50-1000 HP drive Gouge-shaped knives mounted on Dia. 2-4 ft; 12-50 knives; 50- cylindrical drum 500 HP drive

Size class Retained on screen Chips from whole logs Chips from sawmill residuals Green veneer waste Overlength + 45 mm 2% 4% 1% Overthick + 8 mm (hwds), + 10 mm (sftwds) 5 7 0 Accepts + 7 mm 81 71 75 Pin chips + 3 mm 11 14 21 Fines - 3 mm (i.e. caught in pan) 1 4 3

(a) Sketches of chipping action

(b) Assemblage of chips, showing range of dimensions (Carthage).

(c) Chip, with "pin chip" that has broken off it. (Photo courtesy PAPRICAN).

Disc chipper

a) Feed side of 12-knife 8S-inch disc chipper Carthage-Norman). The heavy belt-driven pulley '"1ounted behind the disc acts also as a flywheel. -he upper half of the casing has been removed and stands in the background.

(c) Discharge side of disc chipper fitted with card-breaking pins, for gravity discharge (pulley removed). (Carthage)

(d) Discharge side of disc chipper fitted with vanes for blowing (top) discharge (pulley removed). (Carthage)

Knife arrangements used in disc chipper

(a) Contoured face (Norman design)

(b) Slot (clamp)-mounted knife

(c) Face-mounted knife.

Slot mounted (tclamp-mounted', " " knives slide into the chipper 6.....:: angle of about 38, and can be' from above or below. This type C sharpened by removing material Face-mounted knife: the chipper are placed on the surface of the . disc, and held in place by bolts which pass through the disc (wear) plates on this style of cb:;:, flat and of constant thickness ",-nos.

V-Drum Chippers Another equipment configuration same type of action is the V-drum chi 108), formed by joining two truncaiec together at their small ends. The mounted on the cone surfaces V-shaped space. The log is fed endwise space at right angles to the axis of the chipper.

Drum Chippers Increased attention to the role thickness in pulping has led to an control this dimension also. One equipment designed to do so is a drum where the cutting element is a gouge, bottom and straight sides, all edged, gouges are mounted in rows on the surf drum, and pointed in the direction rotation..The log is presented parallal drum axis. Each gouge strikes the log angles to the grain and removes a chip the length is the width of the gouge. thickness is the depth to which the strikes. Such equipment does not appear "Lrc general use, possibly because of required to sharpen its multiplicity of gouges and the lower capacity of drum

Chipper Operation Chipping is evidently a precision operation. in vhich the choice of chipper size, configuration nd adjustment for getting chips of a given size at given rate from a given raw material involves ne consideration of a good many variables Table 21). Some of the commoner chipper applications are shown in Table 22. Typical settings of a drop-feed disc log chipper in conifer .nd hardwood pulpwood respectively are:

WoodChipperSpecies

Shear strength

parallel to grain Wood specific gravity Tree

growth rate Wood grain

orientation

Piece size

Length

Diameter

Shape

Dryness

Shear strength parallel to grain increases

with wood dryness

Season

Frozen wood requires different chipper

settings Chipper drive power Cutting speed

Chip cut length target (knife projection)

Spout configuration (designed to keep wood

stable while being chipped)

Knife grind angle (face-mounted knife) Knife

clearance angle (slot-mounted knife) Knife-to-

anvil clearance

Table 22. Wood Chipper Applications

Material Chipper Type Additional data ConfigurationLarge cia. (12-24 in) logs

long (10-30 ft) lengths Multiple (4-15) knife disc 6-

10ft dia. with Norman &

slot-mounted knife

arrangement Drop or horizontal feed with

bottom (gravity) discharge A few mills have used V-

drum chippers.

Some have installed rear &

overhead (blowing)

discharge Large cia. (4-12 in) logs

long (4-20 ft) lengths

Multiple (4-12) knife disc 4-

8 ft dia. with Norman. slot-

& face-mounted knife

arrangement Drop feed for mixed lengths

& shorter material;

horizontal feed for

consistently longer logs

Vertical drop feed with

bottom discharge Rather than try to

precision-cut, some mills

reduce with knife or

hammer hogs Sawmill slabs, edge & end

.nrn, & reject lumber Multiple (4-16) knife disc 4-

8 ft dia, with Norman, slot-

or face-mounted knives Horizontal feed with

bottom or overhead

discharge; OR. V-shaped

infeed chute to keep wood

stable while being chippedGood chip size

distributions hard to get

because short, oddlength,

variable-sized pieces hard to

keep stable Green & dry plywood ven-

Multiple (6-8) knife disc 6-7

ft eer waste (without glue)

dia. with face-mounted

knives Multiple (6-8) knife disc 6-7

ft eer, with face-mounted

knivesHorizontal feed with feed

rollers & hold-downs to

permit clean cuts without

shattering Potentially the best chips,

because thickness

controlled. But higher pin

chips can result if feed

system does not hold veneer

securely Oversized chips rejected

from chip screensHammerhog; small dia. (3-4

ft) multiple (4-6) knife disc

chipper; small dia. drum or

V-drum multiple-knife chip

slicer Drop feed with overhead or

bottom discharge Chip slicers normally used

only on overthick chips

rejected by disc thickness

screens

Chipper types

Disc chipper with horizontal

feed for large roundwood.

(b) Disc chipper with drop-feed

residuals (top of casing removed)

(c) V-drum chipper

(Soderhamn).

(f) Slicing rechipper

(Rader)

(g) Diagram of slicing

rechipper (Rader)

CHIP SCREENING

Chips as made whether in the pulpmill or elsewhere, usually vary in their dimensions over a considerable range. Since a digester load of chips will be cooked under the conditions which will give the desired result for average-sized chips. the wider this range of dimensions is, the higher the proportions of overcooked and undercooked chips will be. Moreover, in some types of pulping equipment, oversized and undersized chips cause mechanical problems. Finally, the chips usually contain undesirable foreign matter - eg. particles of bark. sand and dirt - a good proportion of which is substantially smaller than the desired chip size. An operation which would narrow the size distribution by removing oversized and undersized chips along with some of the foreign matter is therefore technically attractive.

However, before embarking on such an operation. a mill should know the characteristics that its chip supply possesses. the economic benefits of improving these characteristics to specified targets (which largely depend on the pulping process and equipment which the mill uses, and the nature of its products), and the cost of doing so, Not all mills need chip screens. In screening (see Process Summary 9), the chips are normally fed first to a screen surface with holes of a size which will retain the oversized and pass the acceptable and undersized, and then to subsequent screen surfaces with holes of a smaller size which will retain the acceptable and pass the undersized.

Fig..Chipping headrig in small-log sawmill (Chip-N-Saw).

Top, the machine; bottom, its profi lr ripping patterns.

Fig. Chip size distribution

before and after screening

PARTICLE SIZE

ui u z ui a: a: ::::> u o o LL o >u z w ::::> a w a: LL

Fig. 1 Chip screens - flat and drum

Gyratory suspended flat screen

(CarthageKone).

(b) Diagram of gyratory

suspended flat screen (Carthage-

Kone)

(c) Vibratory flat screen - fixed

mounting

Fig. Chip screens, disc

(a) Diagram of chip screening/slicing system, V-orientation (Rader)

(b) Chip screening/slicing system, V-orientation

(Rader)

(c) Disc thickness screen, flat

orientation (Black Clawson)

(d) Disc scalping screen

(Rader)

Flat Inclined Gyratory Screen

There are several types of screening equipment which differ in the above regards (Fig. 114). The most widely used is the flat inclined gyratory screen. This has one to three screen decks, each 4 to 10 feet wide and 6 to 20 feet long, inclined at about 40, and base-mounted or suspended by cables from above. A gyratory motion in the horizontal plane is imparted to these decks by an eccentric drive. The commonest configuration is two screen decks, with openings of 11/2 to 2 inches in the upper deck and to Ys inches in the lower one. Sometimes the upper deck is made of a perforated plate, which does not upend and accept overlong chips as much as wire mesh, and a lower deck of wire mesh, which has a greater open area than a perforated plate.

Flat Inclined Vibratory Screen

A flat inclined vibratory screen is the same, except that the agitation imparted to the screens is vibration in the vertical plane. Compare horizontal gyration, this vibration is violent, produces a greater variety of orientations, and is believed to be more eff 'IC' in disengaging undersized material.Drum Screen In this type, the screen is in the form of a inclined and rotated, so that the tumbling a:-xa imparts a succession of orientations to the ; as it passes through. Drum diameter can be 4 to 8 feet, and drum length from 10 to The punched plate openings range in size 3/16 to V2 inch.

Disc Thickness Screen The aim of this type of screen is to pass . retain chips only on the basis of their srnJ::II: dimension, normally their thickness. Its element is a shaft 4 to 8 feet long on whicc llZ" mounted a series of discs spaced at a disul., equal to the desired maximum chip thickness Some 6 to 12 of these shafts are mountec : The chips are fed into the top of this array and agitated by rotating the disks, which are toothed. This agitation, combined with the sharpness of the slot edges, causes each chip to upend so one of its longer dimensions is oriented perpendicular to the slot opening and the other one lengthwise to it.

OperationScreening FunctionEquipment

Options

Flat gyratory

with fixed mount Suspended gyratory

Tumbling drum

Disc (scalping)

Disc (thickness)

Quality parameter Narrow the size distribution of the chip flow by removing largest and smallest 5 ze fractions. (After

removal, overlength and overthick are reprocessed and recycle chips are metered back into flow to

control variability or sent to separate pulpir ; system; fines are normally conveyed to wood waste

fuel)

Description Size Flat and slightly inclined screen with wire Rectangular, 4-12 ft wide, 8-20 ft 0

mesh or punched plate surfaces Openings: overlength 11/2-3 in; fines :. 1/2 in Same as Fixed Gyratory, but Rectangular: dimensions same as Gyratory Circular: dia. up to 15 tt, pie- Seqrne, screen plates Openings same as Fixed Gyratory suspended by cables

Slightly inclined rotating drum with punched plate or

dia 4-12ft. , 8-24 ft wire mesh screen surfaces Discs arranged in staggered pattern on parallel shafts all No. of shafts, 8-24; array 4-8 ft Wide Disc interface opening, 1-4 in rotating in same direction Same as Disc (scalping) Same shaft no. and array width as C s: (scalping)

Screen removal efficiency: A function of feed composition, feed rate, screen surface area

, screen openings, screen motion. Evaluated by measuring chip size distribut tr' before and afer screening.

Removal efficiency of a size fraction = wt. of fraction before - wt. of fraction a"3" wt. of fraction before

Process Summary 9

Disc Scalping Screen

A more rugged version of the disc thickness screen can be used ahead of the screening system to protect it by removing rocks, metal, and wood chunks from purchased or outside-stored chips. In this case the slot width is larger, e.g. from 1 to 5 inches. Processing Chip Screen Rejects Overlong chips are usually reprocessed in a smaller version of the disc-type roundwood chipper, and overthick chips in a chip slicer, the product in both cases being returned to the main chip flow ahead of the screens. The fines are sent to fuel, or else rescreened. Pin chips are metered back into the main chip flow, or sent to a fine particle pulping system.

CHIP CLEANING

If fragments of non-wood contaminants such as rocks, metal and plastics are allowed to accompany the flow of chips to the pulp mill, they will cause wear and damage to the processing equipment, particularly to valves, pumps, and refiners. They may also reduce product quality, the risk of their doing so usually becoming greater as they break up. A large part of these contaminants (though never all) can be kept out of the chip flow in the first place by impressing supplier and mill staffs with the seriousness of these problems. Of the contaminant fragments which nevertheless get in, the fragments which are above the size range of accepted chips are removed by the screening step, and so are most of the fragments below this range.

However, screening does not remove those contaminant fragments which are within the size range for accepted chips, nor those which are below this range but adhere to the accepted chips. Contaminant fragments of these two kinds can only be removed by taking advantage of other differences in physical properties between them and the chips. Magnets A magnet will remove ferrous-metal objects from the chips. It must however be designed and positioned so that it can pull such objects up through the chip flow when the latter is moving at full capacity, in terms both of belt speed and depth of chips on the belt.

Chip Washers

A chip washer will remove contaminants that sink in water, provided that enough dwell time is allowed for them to separate after chips and water have been mixed. The last materials to separate are fine grit and sand, because of their tendency to adhere to the rough surfaces of the chips, and their slower settling rates. Fine particles of wood and bark will also be removed by a chip washer. However, these will not settle out of the wash water, and must be filtered out. Hence to maintain high efficiency in the wash water recycling system, as much of the fines as possible should be removed by screening first.

Pneumatic Cleaners

A pneumatic cleaner separates chips from denser material by suspending the chip flow on a fluidized bed or in a vertical column of air. Its efficiency depends on the difference between the terminal velocity of the chips and that of the contaminant particles; the dwell time in suspension; and the air velocity and turbulence. Hence a pneumatic cleaner is most efficient on larger rocks and pieces of metal, and least efficient on sand and grit. In pneumatic systems. particularly those in areas with large seasonal changes in air temperature and humidity. Centrifugal air cleaners can be designed into pneumatic systems to remove fine particles of wood. sand and grit. Their removal efficiency depends on screen hole size. flow rate. proportion of fines. and chip moisture content.

Fig. Self-cleaning magnet suspended over a belt conveyor carrying chips (Eriez)

Contamination by Plastics

Plastics constitute the one important form contamination for which no removal technique is yet available. Among the most cornmor; sources are sandwich and other food wrappers, the packages in which equipment parts are received, broken plastic parts, and plastic strapping. Education is therefore the most effective tool for avoiding this product quality problem.

Pneumatic chip cleaning devices

(a) Pneumatic cyclone chip cleaner (Rader)

(c) Fluidized-bed pneumatic

separator (Triple-S Dynamics)

(b) Vertical column

pneumatic separator

(Rader).

Cleaning Full-tree Chips

Full-tree chips present a special problem. They contain a high percentage of foliage, dirt and bark, and much of this bark is still attached so firmly to the chip that it is not removed merely by screening and washing. Without further treatment, therefore, full-tree chips can only be used in large proportion for pulps in which bark is no problem or for pulps made by processes which effectively decolorize bark fragments, e.g. fully-bleached kraft. For many other pulps e.g. semi-bleached kraft for newsprint, unbleached sulphite, and refiner pulps - their use is limited.

Fig. Chip washing system (Defibrator). Inset: closeup of screw drainer.

It is possible to reduce the bark content of full-tree chips by methods which loosen the bark-to-wood bond before washing. One such method comprises: Weakening the attached bark either by steaming the chips for 6 to 10 minutes. or by storing them for several weeks. Detaching bark and foliage and breaking them into small fragments by vigorously agitating the chips in water, e.g. in a pulper. Separating these fragments from the chips by washing them through a screen with suitably-sized holes. Separating the fragments from the water. pressing them for fuel. and recycling the water.